1. Field of the Invention
The present invention relates to an apparatus for forming an image without causing any shift in position of the image due to inclination of a polygon mirror.
2. Description of the Related Art
In an image forming apparatus for performing a scanning operation using a laser beam by a polygon mirror, a writing position is changed by the inclination of a deflecting face of the polygon mirror.
Accordingly, a correction optical system composed of a cylindrical lens, a toroidal lens, etc. is disposed in the image forming apparatus of this kind to correct the writing position. In a black-and-white laser printer, a change in pitch of a scanning line is removed as much as possible by improving the accuracy in processing of the inclination of the polygon mirror. Thus, the accuracy in dot position in a cross scanning direction is set not to be practically influenced by the above change in pitch.
In a laser beam printer of a four drum system, writing positions are aligned with each other by conforming phases of a light beam with respect to individual rotary polyherons to each other. Such a laser printer is proposed in Japanese Patent Application Laying Open (KOKAI) No. 62-242471.
When the above correcting optical system is used in the image forming apparatus, it is possible to reduce image noises caused by an irregular pitch.
However, there is a case in which field curvature is caused in a certain place in which a lens for correction is arranged in the correction optical system. Accordingly, the arrangement position of the lens for correction is limited and it is impossible to make the correction optical system compact so that the entire apparatus is large-sized.
The change in pitch of a scanning line can be restricted by improving the accuracy in processing of the inclination of a polygon mirror face without using the correction optical system.
A black-and-white image is formed by improving the accuracy in processing of the inclination of the polygon mirror face. However, when images overlapped and transferred to each other to form a color image, a slight shift in color with respect to color images tends to be caused by the inclination of polygon mirror face.
The accuracy in processing of the inclination of the polygon mirror face must be greatly improved not to cause this shift in color in the case of the color image, thereby increasing manufacturing cost.